Stiffened composite structures are structures that are constructed of composite materials, such as fiber reinforced composite materials, and typically include some form of structural frame that carries a skin. Some modern aircraft fuselages are examples of stiffened composite structures that include a skin operatively coupled to frame members that extend circumferentially around and that are spaced longitudinally along the inside of the fuselage and stringers that extend longitudinally along and that are spaced circumferentially around the inside of the fuselage. Typically, stiffened composite fuselages are constructed utilizing an inner mold line layup mandrel that includes stringer cavities, or forms, that extend longitudinally along the mandrel. To prepare the mandrel, resin may be hand-applied to the stringer cavities, and stringers may be hand-aligned into position in the stringer cavities. After several stringers have been placed in respective stringer cavities, they are vacuum compacted in batches, utilizing a gas-impermeable flexible sheet of material that extends across the outer surface of the mandrel and that is sealed to the mandrel around the outer perimeter of the stringers that are being compacted. A vacuum is then applied between the sheet of material and the mandrel to compress the stringers to the mandrel. This process is referred to in the aerospace industry as “bagging.” The bagging and compacting process can be labor and time intensive.